1. Field of the Invention
The present invention relates to a hydraulic braking system for use in an automotive vehicle, and more particularly to a hydraulic braking system which has a hydraulic booster for actuating a master cylinder in response to operation of a manually-operated member with a hydraulic power pressure supplied from a power source, and in which a first group of wheel brake cylinders communicates with the master cylinder and a second group of wheel brake cylinders communicates with the hydraulic booster.
2. Description of the Prior Art
In conventional service braking systems for an automotive vehicle, there are provided a plurality of hydraulic circuits connecting wheel brake cylinders mounted on road wheels with a hydraulic braking pressure generator such as a master cylinder, so that when one of the hydraulic circuits is broken, braking operation is performed by the rest of the hydraulic circuits. In general, a tandem master cylinder is used in a conventional dual circuits system.
In order to reduce a force required to operate a manually-operated member, for example a depressing force applied on a brake pedal in braking operation, the hydraulic braking system is provided with a servo unit which is referred as a servo or a booster and which utilizes compressed air, intake manifold vacuum (for a vacuum booster), or hydraulic pressure (for a hydraulic booster) as a power source. The hydraulic booster is a booster which actuates the hydraulic braking pressure generator such as the master cylinder by the hydraulic power pressure supplied from the power source in response to depression of the brake pedal.
It has been proposed to employ the hydraulic booster, in the hydraulic braking system, as a dynamic hydraulic braking pressure generator in addition to the master cylinder In other words, a hydraulic pressure generated by the hydraulic booster in response to depression of the brake pedal is applied directly to one of the hydraulic circuits. For example, as shown in Japanese Patent Laid-open Publication No. 59-227552, the hydraulic pressure generated by the hydraulic booster is applied to rear wheel brake cylinders in a front-rear dual circuits system in order to reduce the stroke of the brake pedal.
However, in the above-described hydraulic braking system, the hydraulic pressure generated by the hydraulic booster is applied to each of the rear wheel brake cylinders as it is, so that if any of the rear wheel brake cylinders or a hydraulic circuit communicating therewith is damaged, the brake fluid filled in the hydraulic booster and the hydraulic circuit communicating therewith may leak therefrom. In this case, with a brake force applied sufficiently on the normal front road wheels, necessary braking operation may be performed. However, since the hydraulic pressure generated by the hydraulic booster disappears, boosting operation is not performed, so that a large depressing force shall be applied on the brake pedal.